1. Field of the Invention
The invention relates to the field of hydraulic systems and, in particular, to a system for the control of aerodynamic surfaces and the like on aircraft.
2. Description of Related Art
The hydrostatic actuator is based on applying motive power driven hydraulic pressure directly to the hydraulic cylinder piston without going through a valve. The electro-hydrostatic actuator (EHA) utilizes a pump driven directly by an electric motor to drive the hydraulic piston or motor. The EHA differs from the conventional hydraulic servo actuator in using a motor controller to control both motor speed and direction in accordance with the actuator position sensor and rate command. Applying the EHA to aircraft applications also the posted additional challenge of driving unbalanced piston area cylinder typical found in the aircraft actuation application and the fast response that is required to actuate the flight control surface. There are several implementation options that exist.
For a balanced piston area cylinder, the flow circuit is simpler and can be implemented using a set of back-to-back check valves to handle the minor makeup flow that may have resulted from internal or external leakages. If the low pressure side is below the reservoir pressure, the fluid is flowed through the check valve and the pressure is maintained at the reservoir pressure level. Some tests have shown the stiffness characteristics of this arrangement is acceptable with the cylinder is charged in few excitation cycles and both fluid columns of the cylinder contributed to the stiffness. Such a system is disclosed in U.S. Pat. No. 6,625,982 Electronically Controlled Hydraulic Actuating System by Dominique Alain Van Den Bossche and etc. However, this approach only works for a balanced piston area cylinder, which is not desirable for space limited aircraft applications.
For the unbalanced piston area cylinder, the back-to-back check valves have to be replaced with an anti-cavitations valve. The anti-cavitations valve is a set of pressure actuated check valves. The low pressure side is forced to open to allow fluid flow out from the reservoir to make up the lower flow coming from the smaller piston area. However, this opens the low pressure side to the fluid volume of the reservoir thus lowering the actuator stiffness that is critical for most all flight control actuation applications. Such a system is disclosed in U.S. Pat. No. 6,543,223 Drive Device by G. Muschong, et al. Such a system is also disclosed in U.S. Pat. No. 5,575,150 Stiffness Enhanced Electro-hydrostatic Actuator by Kurt W. Vieten and etc. attempting to block the cylinder flow path in order to maintain stiffness during load cycling.
Thus, it is a primary object of the invention to provide a hydraulic cylinder control system.
It is another primary object of the invention to provide a hydraulic cylinder control system that couples the hydraulic pump directly to the cylinder without the use of a servo-valve.
It is a further object of the invention to provide a hydraulic cylinder control system, which includes a backup system.
It is a further object of the invention to provide an electro-hydrostatic actuator with enhanced stiffness.
It is a further objective of the invention to provide an electro-hydrostatic control system of un-balanced cylinder piston area.
It is a further objective of the invention to use a common hydraulic system to backup multiple electro-hydrostatic actuators.
It is a further objective of the invention to backup the electro-hydrostatic actuator using modulating valves.
It is a further objective of the invention to maintain low parts count for high reliability.